The present invention relates to the protection of d.c. motors, particularly against undue current increases in the case of excessive load (torque) requirements for, e.g., a reeling or paper feed motor in printers.
A protection of the type referred to above is needed in those cases in which the motor may experience an unexpected overload so as to prevent damage to the motor. Printer motors may experience an excessive load, for example in the case of paper jamming. The usual remedy here is of a mechanical nature such as a slip clutch which decouples the motor from the paper feed drum in the case the drum is blocked. Such clutches or couplings respond to a particular torque and decouple (slip) regardless of the cause. In lieu of a mechanical clutch magnetic couplings have been used. However, these couplings are disadvantaged by the fact that in the case of an overload no torque is transmitted, and full engagement is present even if the load has ceased to turn.
It has been proposed to use a.c. synchronous motors for a reeling and feed operation in printers. However, this type of motor yields only a very low torque when operated (as is usual in printers) in a stepping mode. For this reason, d.c. motors are clearly preferred because even when operated as a stepping motor the developed torque is high.
German printed patent application 25 17 766 (IPC B 41 J 29/54) proposes an electrical protective circuit for matrix printers, for monitoring a single function. In order to recognize any interference sufficiently fast, a resettable timer is associated with the device or element to be monitored, and the timer is triggered anew with each initialization of this device or element. The timer meters a period in excess of the regular operation time of that device or element and will, thus, respond when the repeat operation of the device or element does not recur within that period. This approach is not adequate for monitoring a motor as the current increase in the latter is, in the case of jamming, faster than a metered regular operation of e.g. the paper feeder. In essence, this approach is not applicable in those cases in which two or more mechanical operations are linked, and the disturbance may arise in the first one, blocking the succeeding one or ones, but its or their function is covered by time-metering.